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The Deepwater Horizon (DWH) incident was the largest offshore oil spill in the history of the United States, contaminating surface waters, the water column, deep-sea corals and benthos, nearshore and coastal ecosystems, and natural resources across 5 states and an ocean area of more than 112 000 km 2 in the Gulf of Mexico (GoM). Protected marine species -sea turtles and marine mammals, in particular -were a main focus of the DWH Natural Resource Damage Assessment (NRDA). The DWH spill overlapped in time and space with sea turtle and marine mammal habitats and life stages throughout the northern GoM. Thus, the DWH NRDA Trustees (2016; www.gulfspillrestoration.noaa.gov/restoration-planning/gulf-plan/) performed several activities to assess adverse effects of oil exposure on sea turtles and marine mammals to quantify the full extent and nature of the impacts to these taxa across the region. A synopsis of the Trustees' assessment activities and conclusions is presented in the DWH NRDA Programmatic Damage Assessment and Restoration Plan (DWH NRDA Trustees 2016). This Theme Section presents several of these specific sea turtle and marine mammal assessment activities and associated findings. This Overview provides a context for the Theme Section papers, introduces basic NRDA concepts and discusses generally why and how protected marine species were assessed in the DWH NRDA.

Several recent oil spills in the United States have had the potential to impact large numbers and multiple populations of cetaceans (whales and dolphins) and pinnipeds (seals and sea lions), namely the Macondo-252/Deepwater Horizon oil spill from April 2010, the Texas City Y event in March 2014, and the Refugio Beach oil spill in May 2015. In each of these spills, the National Oceanic and Atmospheric Administration (NOAA) and partners have engaged in significant activities during and following the spills, to both respond effectively to minimize impacts and assess the effects of oil spills on marine mammals. Experience gained during these spills has led to improved preparedness for future events with potential involvement of marine mammals. NOAA's National Marine Fisheries Service (NMFS) has developed the “National Pinniped and Cetacean Oil Spill Response Guidelines,” which are available online. These guidelines provide a broad national overview of response activities, a proposed organizational structure, and considerations to identify, recover, treat, and sample oiled and potentially oiled marine mammals. Further development of a response framework includes regional preparedness plans for marine mammals, based on the national guidelines but with regionally appropriate modifications to reflect local considerations, including species likely to be impacted, geographic concerns, and understanding of local cultural practices. Efforts are underway to improve training opportunities through a national exercise plan and to track trained and qualified individuals for potential deployment. For assessment, NMFS and NOAA's National Ocean Service (NOS) have partnered to develop the “National Marine Mammal Oil Spill Assessment Guidelines” to facilitate early, efficient, and effective assessment of impacts from oil spills on marine mammals as part of the Natural Resource Damage Assessment process. This paper will outline the current status of these products and their role in marine mammal assessment and response in the U.S., raise awareness of marine mammals within oil spills, and identify potential resources for marine mammal response and assessment in other countries.